1. Field of the Invention
The present invention is directed to a directly heated thermionic flat emitter of the type having an emission surface divided by slots with a number of interconnects, and having a terminal lug at a periphery of the emission surface for connection to a power lead.
2. Description of the Prior Art
Thermionic flat emitters of the aforementioned type as disclosed, for example, in U.S. Pat. No. 6,115,453 and German OS 100 16 125 are utilized in X-ray tubes, particularly in rotating bulb X-ray tubes. That part of the emitter forming the emission surface is usually fashioned circular or disk-like and is composed of a thin tungsten sheet approximately 100 xcexcm thick. The emission surface is heated to above 2000xc2x0 C. in order to emit electrons during operation. Emission of electrons then occurs everywhere where an adequately high electrical field extracts the emitted electrons. The electron optics is thereby determined by all potential-carrying elements in the proximity of the emitter. The seating of the emitter relative to the cathode head has a particular influence on the shape of the focal spot as well as on the distribution of the focal spot on the anode. In order to avoid shorts between the emitter and the cathode head, the bore in the cathode head is selected approximately 0.4 mm larger than the diameter of the emitter. It has been shown that the gap of approximately 0.2 mm that thereby exists at each side between the emitter and the cathode head bends the electron trajectories in the edge region of the emitter. This effect has a negative influence on the focal spot occupation and thus ultimately on the image quality of the X-ray image produced with the tube. This disadvantage can be partially compensated by placing the emitter deeper in the head but cannot be entirely eliminated.
Placing the emitter deeper leads to another negative effect, namely that the electrons are emitted proceeding from the back side of the emitter.
These two effectsxe2x80x94the bending of the electrical field and the emission of the electrons from the back side of the emitterxe2x80x94contribute to a halo in the focal spot occupation of the rotating bulb tube. This halo ultimately degrades the image quality in the practical utilization of the rotating bulb tube, for example in computed tomography.
An object of the present invention is to eliminate the aforementioned disadvantages in a directly heated thermionic emitter of the type initially described that is employable, in particular, in rotating bulb X-ray tubes. In particular, a bending of the electron trajectories in the edge region of the emitter and an electron emission from the back side of the emitter are to be avoided.
The above object is achieved in accordance with the invention in a directly heated thermionic emitter having an emission surface which is divided by slots into a number of interconnects. A number of segments surround a periphery of the emission surface. The segments are not connected to each other and are connected to interconnects at the peripheral region of the emission surface by webs. The webs are spaced and dimensioned so that no current flows from the interconnects to the segments, and so that there is no appreciable heat transfer from the emission surface to the segments.
As a result of the inventive proposed arrangement of segments, an additional, non-emitting ring is formed around the emitter that causes the equipotential surfaces to be undistorted at the edge of the actual emitting surface of the emitter. The ring creates a larger distance between the gap at the cathode head and the outer edge of the emission surface of the emitter, as a result of which the influence on the electron trajectories is kept negligibly small. The additional ring created in this way also effects a reduction of the field strength at the back side of the emitter, so that fewer electrons are extracted from the back side of the emitter.